1. Field of the Invention
The present invention relates to a punch unit for punching sheets, a sheet post-processing apparatus having the punch unit, and a method of punching sheets. More particularly, the invention relates to a punch unit that can deal with a skew of sheets, a sheet post-processing apparatus that has the punch unit, and a method of punching sheets.
2. Description of the Related Art
In recent years, a sheet post-processing apparatus is provided at the sheet-ejecting unit of an image forming apparatuses, in order to perform sheet post-processing, such as stapling or punching, on sheets having images formed on them.
Punching for filing sheets is performed by a punch unit on the sheets transported to the punch unit. If the sheets are skewed, or inclined to a prescribed straight line, the holes made in the sheets by punching will be skewed, too. That is, if the sheets are skewed and inserted in an inclined state, errors will be made in terms of the position and shape of the holes. Consequently, the position accuracy of holes cannot be achieved.
Hitherto, various systems have been proposed to correct a skew of sheets. In one system, the leading edge of a sheet is detected by a sensor, the degree of skew is calculated, and the punch unit is inclined (see, for example, Jpn. Pat. Appln. Laid-Open Publication No. 2004-9245 and Jpn. Pat. Appln. Laid-Open Publication No. 9-244325). In another system, a CCD line sensor is used to find the degree of skew from the feeding rate and the deviation of a side of the sheet (see, for example, Jpn. Pat. Appln. Laid-Open Publication No. 10-194557).
If the skew of a sheet is detected at the leading edge, however, it is impossible to correct the skew that occurs after the leading edge of the sheet has passed the sensor and before the trailing edge passes the sensor. If longitudinal registration is performed when the sensor detects this skew at the trailing edge, there will be no time to correct the skew.